feat(if-else): add if-else access control

[HansBhatia]

Closes #40

[enitrat]

added if_else #40

A good practice is to link your PRs to existing issues with predefined keywords, like Closes: e.g.
Closes #40

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/using-keywords-in-issues-and-pull-requests

[enitrat]

Ready to merge after addressing the last small comments ;)

[HansBhatia]

Changes made! I cannot merge :)

[enitrat]

I just realized @HansBhatia, you forgot to add a .md page under the src directory with a short explanation of if-else and the code written

[HansBhatia]

@enitrat I believe the changes are made!

[enitrat]

Great!
You also need to add the chapter to the SUMMARY.md file for it to appear in the book.
Also, I renamed "control" to "log", given that our contract is no longer controlling anything, but just logging.